Natural prostaglandins have a unique structure and are known as compounds showing a broad physiological activity in a small amount, attracting interest of many organic synthetic chemists, and it is desired to purely synthesize natural prostaglandins in which there is a difficulty for naturally obtaining them in a large amount in order to further study a minute biological activity, and furthermore a lot of synthetic work has been done from such reasons that a lot of structural analogues from natural prostaglandins can be synthesized and these prostaglandin analogues are interesting from the aspect of a drug development.
As one of the representative preparation processes of prostaglandins there is the process in which the omega side chain is introduced by affecting an organometallic reagent to an .alpha., .beta.-unsaturated cyclopentenone. Especially is well known such a process that a vinylstannane compound having an aimed omega side chain structure or its corresponding vinyliodide compound are converted to the vinylcopper complex to carry out the conjugate addition to the .alpha., .beta.-unsaturated cyclopentenone [U.S. Pat. No. 4,777,275 (JP, A, S63-316786), U.S. Pat. No. 4,543,421 (JP, B, S63-44743)]. Typically, a vinylstannane compound is prepared by affecting tin hydride under a radical condition after the addition of an acid unstable protective group such as a trialkylsilyl group to a free hydroxyl group which a terminal alkyne of a starting material has [U.S. Pat. No. 4,087,447 (JP, A, S53-108929)]. The conversion to the vinyliodide is done by treating the vinylstannane compound with iodine. An alkylmetal reagent is reacted with the obtained vinylstannane compound or its corresponding vinyliodide compound to give a vinylmetal compound, and subsequently a cuprous (I) salt is added to give a vinylcopper complex. Thereto is added an .alpha., .beta.-unsaturated cyclopentenone to give the 1,4-conjugate addition product. Since one more step is necessary to obtain a vinyliodide compound from a vinylstannane compound and there is no difference between the vinylstannane compound and the vinyliodide compound, there is no special advantage to derivatize it into the vinyliodide compound. Therefore, a vinylmetal compound via a vinylstannane compound is advantageous, though there is an unfavorable problem that a geometrical isomer accompanies in case of preparing the vinylstannane compound by the above process.